In the telephone system the principal method of connection between the exchange and the subscriber equipment is by a two wire telephone line. In the USA the two wires are called R(ing) and T(ip), while in other areas they are called A and B.
In normal operation wires R and T will be at different potentials wth respect to ground. For a given system practice there will be maximum values that these voltages can assume in normal operation. The maximum positive and negative voltages that wires R and T can reach are not necessarily the same. Also the voltage differential between the wires may not be as large as the sum of maximum positive and negative excursions of the two line wires with respect to ground.
Telephone equipment with components connected from any of the wires to ground and between the wires must have adequate voltage ratings to withstand the voltage potentials which occur in normal operation. Typically, a battery voltage of about 50 V will be applied to the line, and during ringing an additional AC voltage of 100 V.sub.rms (141 V peak) will be applied. Under these conditions a peak voltage of 50+142=192 V will occur. After allowing for tolerancing, the equipment might be designed for a maximum voltage of 200 V in normal operation.
Under disturbing (abnormal) voltage conditions, caused by lightning, power line contact and induction from electrical machines, the above components must be protected against failure from these dangerous overvoltages which can occur on the line wires.
FIG. 1 shows an overvoltage protection arrangement which uses two voltage limiting devices, P1 and P2, connected from each line wire to ground. From the above example, these protectors must not limit the voltage from reaching 200 V, otherwise the normal operation of the telephone system would be impaired. The voltage limiting level of the protectors depends on the protection technology and the type of disturbing voltage that occurs. Typically, the limiting voltage of the protector passing 200 V might be 300 V under AC conditions and 350 V for fast rising/high current lightning impulse conditions.
To minimize the time for which the high voltage exists, voltage switching (crowbar) protectors are used. The characteristic and voltage limiting performance of a voltage switching protector is shown in FIG. 2. When the impulse finishes, the protector in its low voltage state will still have the wire current from the exchange battery flowing in it. It is most important that the protector switches off at this current, otherwise the line would remain shorted to ground preventing normal operation. In thyristor voltage switching protectors, this switch off current is controlled by shorting dots (see S. W. Byatt & R. A. Rodrigues: British Patent Application GB 2113 907 A, "Reverse-Breakdown PN Junction devices", published August 1983). Unfortunately, the presence of these dots slows the protector switching and this contributes to the limiting voltage increase from AC to impulse conditions (300 V to 350 V ). Typically the line DC is about 50 mA, so protectors are made with minimum switch off currents of about 150 mA.
In summary, for the example quoted, equipment designed to operate up to 200 V peak requires components rated for at least 350 V. But, as the disturbing voltage conditions are not controlled, for long service life the components may need even higher voltage ratings.